selective image encryption

Hello Friends,
I got a problem and need your help. I am doing my master's thesis
in "Selective Image Encryption". My guide suggested me to work on
java. I have learnt the java tutorial and able to encrypt the
whole image. But I am unable to perform selective image encryption.
Can you put some light on doing Selective Encryption in java.
I will be thankful to you.
Please do help me...